1. Field of the Invention
The present invention relates to image pickup apparatuses, radiation imaging apparatuses, and image pickup systems which are suitable for medical diagnosis or industrial nondestructive inspection, and, more particularly, to an image pickup apparatus, a radiation imaging apparatus, and an image pickup system capable of reading a signal output from a planar detector as digital data using low-noise reading circuitry.
2. Description of the Related Art
In current years, radiation imaging apparatuses using a planar detector (hereinafter referred to as a Flat Panel Detector (FPD)) made of a semiconductor material are in practical use as image pickup apparatuses used for a medical image diagnosis or a nondestructive inspection using an X-ray. Radiation imaging apparatuses using the FPD can perform digital photographing by converting radiation such as an X-ray that has passed through a test object such as a patient into an analog electric signal with the FPD and converting the analog electric signal into a digital image signal. FPDs may be used as digital image pickup apparatuses for still image capturing (general image capturing) and moving image capturing (fluoroscopy).
U.S. Pat. No. 6,538,591 and Japanese Patent Laid-Open No. 2000-188724 disclose signal output circuits (reading circuits) used in image pickup apparatuses as those described above. U.S. Pat. No. 6,538,591 discloses a signal transfer apparatus for sequentially reading parallel signals from a plurality of computation amplifiers as a serial signal using a reading switch and converting a serial signal transmitted via an output buffer amplifier into digital data with an analog-to-digital (A/D) conversion circuit (ADC) unit. Japanese Patent Laid-Open No. 2000-188724 discloses a circuit obtained by disposing a programmable gain amplifier (PGA) at a stage prior to the output buffer amplifier in the signal transfer apparatus disclosed in U.S. Pat. No. 6,538,591. The signal transfer apparatus disclosed in U.S. Pat. No. 6,538,591 and the circuit disclosed in Japanese Patent Laid-Open No. 2000-188724 have an excellent signal-to-noise ratio (S/N) property and a high reading speed, and are suitable for the increase in the number of pixels.
Signal output circuits disclosed in U.S. Pat. No. 6,538,591 and Japanese Patent Laid-Open No. 2000-188724 are required to achieve further reduction in chip size and a higher degree of integration. Accordingly, the further reduction in size of elements used in the signal output circuits is needed. However, when small elements are used in signal output circuitry, 1/f (flicker) noise occurs. The 1/f noise can be reduced by increasing an element size. However, the opposite is also true. That is, when small elements are used, 1/f noise cannot be reduced. In such a case, 1/f noise caused by an output buffer amplifier, a PGA or an ADC unit, which are disposed at stages subsequent to a reading switch, generates a low-frequency artifact on an image. In particular, since radiation imaging apparatuses require a wide dynamic range, it is necessary to use a low-noise reading circuit in these radiation imaging apparatuses.